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K [0, +∞[ R > 0 B(0, R) ⊇ K <br />

∞<br />

<br />

<br />

sup <br />

n + x<br />

n3<br />

<br />

<br />

<br />

+ x<br />

≤<br />

+∞ n + R<br />

n3 =<br />

+∞<br />

∞ 1 1<br />

+ R < +∞,<br />

n2 n3 n=1<br />

x∈K<br />

n=1<br />

[0, +∞[ [0, +∞[<br />

[0, +∞[<br />

<br />

∞<br />

n + x<br />

sup <br />

n3 + x −<br />

N n + x<br />

n3 <br />

<br />

<br />

≥ sup <br />

+ x<br />

<br />

2N n + x<br />

n3 <br />

<br />

<br />

<br />

+ x<br />

x≥0<br />

n=1<br />

n=1<br />

n=1<br />

x≥0<br />

n=1<br />

n=N+1<br />

sup xj <br />

<br />

2N<br />

n + x<br />

sup <br />

n3 <br />

<br />

<br />

<br />

+ x<br />

≥<br />

2N<br />

1 = N − 1,<br />

x≥0<br />

n=N+1<br />

N → +∞<br />

∞<br />

<br />

<br />

0<br />

n=1<br />

n=1<br />

n=N+1<br />

e−n√x n2 [0, +∞[<br />

+ 1<br />

∞<br />

<br />

<br />

e<br />

sup <br />

<br />

−n√x n2 <br />

<br />

<br />

<br />

+ 1<br />

≤<br />

n=1<br />

x≥0<br />

n=1<br />

∞<br />

n=1<br />

1<br />

< +∞,<br />

n2 <br />

∞ <br />

log 1 +<br />

n=1<br />

x<br />

n2 <br />

[0, +∞[<br />

[0, +∞[ <br />

1 ∞ <br />

log 1 + x<br />

n2 ∞<br />

<br />

= (1 + n 2 <br />

) log 1 + 1<br />

n2 <br />

− 1 .<br />

s > 0 log(1 + s) < s g(s) =<br />

log(1 + s) − s g(0) = 0 g ′ (s) = 1<br />

1+s − 1 < 0 s > 0 g(s) < g(0) ≤ 0 s > 0 <br />

K <br />

+∞ <br />

<br />

sup log 1 + x<br />

n2 ∞<br />

1<br />

≤ sup |x| < +∞,<br />

x∈K n2 n=1<br />

x∈K<br />

K <br />

[0, 1] <br />

1 <br />

log 1 +<br />

0<br />

x<br />

n2 <br />

dx = n 2<br />

1+1/n2 log y dy = n<br />

1<br />

2 [y log y − y] y=1+1/n2<br />

y=1<br />

= n 2<br />

<br />

1 + 1<br />

n2 <br />

log 1 + 1<br />

n2 <br />

− 1 − 1<br />

<br />

+ 1 ,<br />

n2 <br />

n=1

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